The present invention relates to the general field of thrusters or rocket engines for delivering thrust for driving aerospace vehicles such as missiles, launchers, or indeed satellites, using the principle of propulsion by reaction or by ejecting gas. The invention relates more precisely, but not exclusively, to the fixed diverging portion nozzles fitted to solid propellant thrusters.
A solid propellant thruster is mainly constituted by a shell containing a block of powder (propellant), an ignitor, and a nozzle with a fixed diverging portion. The block of powder is pierced by a channel lying on the axis of the thruster that acts as a combustion chamber. The ignitor ignites the powder at one end of the shell and combustion of the propellant propagates from the front towards the rear of the thruster. The powder burns at a predefined rate, thereby producing combustion gas that is expelled via the nozzle.
The throat section of the nozzle makes it possible to control the combustion of the block of powder so as to maintain a desired pressure in the combustion chamber while producing the expected thrust. Thus, for a thruster delivering thrust at a single discharge rate, the throat section of the nozzle is unvarying and predetermined as a function of the desired level of thrust.
Nevertheless, having recourse to a single unvarying throat section is not suitable for a thruster that has two rates of operation (typically a high discharge rate and a low discharge rate).
In order to mitigate such a drawback, it is known to provide the nozzle with a throat section that is variable. In practice, a needle that is movable in translation is received inside the nozzle. The position of the needle in the flow of combustion gas serves to determine the flow section for the gas passing through the nozzle, thereby adjusting the gas ejection section to the operating rate of the thruster.
Nevertheless, the positioning systems associated with the movable needle are active systems requiring an external source of energy in order to operate, thereby making more complex the device for modulating the ejection section and increasing the risk of the device failing or operating in faulty manner. There thus exists a need to provide modulation of the ejection section in a nozzle without using an external source of energy so as to cause the combustion gas to pass through the main ejection section of the nozzle during a stage of operation with a high level of pressure in the thruster (corresponding to operating at a high discharge rate) and then, during a stage of operation at a lower level of pressure in the thruster (corresponding to operating at a low discharge rate), causing the gas to pass via a secondary ejection section positioned in the nozzle, in order to optimize the thrust coefficients during the various stages of operation (high discharge rate and low discharge rate).